The present invention relates generally to aircraft voice communication radios such as are normally employed in two-way communication with air traffic control facilities. More particularly, the invention relates to means for preventing interference of the type which arises when two or more radios are transmitting on the same channel frequency.
Two-way voice communications between aircraft and groud-based air traffic control facilities are generally carried out by means of VHF communications transceivers, customarily termed "COM" radios. Typically, COM radios employ amplitude modulation (AM) and operate in a frequency band from 118.0 MHz to 135.95 MHz on any of a plurality of selectable discrete channel frequencies, e.g., on any one of 360 defined channels, with a 0.05 MHz channel spacing.
At each radio location, whether within an aircraft or at an air traffic control facility, there is a manually-operable push-to-talk (PTT) switch for activating the transmitter circuitry and de-activating the receiver circuitry. Thus the normal or stand-by mode is the receiver mode. In the case of a hand-held microphone, the PTT switch is normally included within a microphone housing including both a microphone element and the PTT switch. A microphone cord includes both audio and PTT control conductors, and terminates in a suitable microphone plug. In other cases, a so-called boom microphone element is attached to a headset assembly, and the push-to-talk switch is located elsewhere, for example, on the aircraft control yoke. In either case, the transmitter circuitry can be activated any time at the discretion of the radio operator, such as the aircraft pilot.
Proper radio operating technique is to always listen before transmitting on a particular channel frequency, in order to ensure that the particular channel frequency is not already in use. However, in practice, proper technique is not always followed, and occasionally pilots transmit on a channel frequency at the same time someone else, either another aircraft or a ground facility, is transmitting on the same channel. This is known as "stepping on" the other transmission. The consequence of stepping on a transmission in many cases is that neither transmission is intelligible at the receiving station. Thus, the particular radio frequency channel is temporarily rendered useless. This is particularly so since amplitude modulation (AM) is employed in the type of aircraft communications radio here concerned, rather than frequency modulation (FM). FM communications systems exhibit a "capture effect" whereby the strongest signal presented to a receiver is heard clearly, to the complete exclusion of other, weaker transmissions. However, in AM communication systems, all transmissions on a particular channel frequency are heard in the receiver causing mutual interference. Moreover, since the actual carrier frequencies of several transmitters nominally on the same channel frequency are rarely in fact identical, but rather can differ by up to several kHz, hetrodynes or beat notes in the form of one or more squeals are normally heard in an AM receiver when more than one transmitter is on the same channel frequency, increasing the effect mutual interference.
In many two-way communication systems, one transmission "stepping on" another may be no more than a minor annoyance. However, in the context of an aircraft communication system, one transmission 37 stepping on" another is potentially hazardous. This is particularly so because, in busy air traffic control situations, a controller may be communicating in rapid sequence with a number of aircraft employing brief messages and terse phraseology to maximize the amount of information which can be communicated in short period of time. At aircraft speeds, situations develop rapidly, and it is important that aircraft communications proceed in a smooth fashion. The potentially hazardous consequences of a missed communication during a critical phase of flight will be well appreciated.
In the field of radio communication in general, a variety of approaches have been employed for avoiding mutual interference between multiple radio transmitters. For example, automatic contention-resolving systems for use where a number of communication units share a common channel or radio frequency are disclosed in the following U.S. Patents: Doremus et al U.S. Pat. No. 2,731,622; Walker U.S. Pat. No. 2,731,635; Patterson U.S. Pat. No. 4,013,959; White et al U.S. Pat. No. 4,199,661; and Gable U.S. Pat. No. 4,259,663. Similar systems have also been employed in operator-controlled communication systems, such as are disclosed in the following representative U.S. Patents: Latour U.S. Pat. No. 1,612,448; Halstead U.S. Pat. No. 2,577,751; Dorff U.S. Pat. No. 2,662,974; Nordahl U.S. Pat. No. 2,691,723; Lutz U.S. Pat. No. 2,858,420; Yamato et al U.S. Pat. No. 2,932,729; and Lunden U.S. Pat. No. 3,938,156. Considering, for example, the Dorff U.S. Pat. No. 2,662,974 in greater detail, the Dorff patent discloses a technique for preventing mutual RF interference between a number transmitters operating on the same frequency, by selectively inhibiting transmission when the channel is sensed to be in use. In the Dorff system, a monitor receiver, tuned to the transmission frequency, senses the presence of any RF carrier and either inhibits or enabled keying of the transmitter at its location through several interactive relays.